Active and passive components of chloride transport in the rat proximal convoluted tubule.

Rat proximal convoluted tubules were perfused in vivo to examine the active and passive components of chloride absorption. Chloride flux was a linear function of the transepithelial electrochemical driving force, yielding a permeability coefficient of 20.6 X 10(-5) cm/s. In the absence of an electrochemical driving force, chloride absorption persisted at the rate of 131 peq/mm X min, thus demonstrating active absorption of chloride. Addition of luminal cyanide to tubules absorbing chloride inhibited net chloride absorption. In tubules perfused with a low luminal chloride concentration in which there was net chloride secretion, addition of luminal cyanide increased the magnitude of net chloride secretion. These studies demonstrate that transepithelial chloride transport involves two components: a passive paracellular flux and an active transcellular flux. Cyanide affects net chloride flux by inhibiting active transcellular chloride absorption.